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Body Forms “Industrial” Lubricant for Metal Hip Implants

A new study shows that a lubricating layer forms naturally in the joints of metal-on-metal hip implants. This solid layer, made mostly of carbon, is more like an industrial lubricant than joint fluid. The finding may help researchers design longer-lasting metal hips for treating arthritis and other joint disorders.

Arthritis is a painful condition often characterized by swollen joints. Over time arthritic joints can become damaged and too painful to use. When this happens, surgeons resurface or replace the joint with an implant.

Hip implants can be made of several materials, including plastic. But implants with metal ball and socket parts (metal-on-metal) have become increasingly popular.

All-metal joints aren’t designed with lubrication. But in the body, a thin, slippery layer develops between the metal ball and socket. Researchers thought that the layer must be made of proteins, like the fluid in normal joints.

A team of NIH-funded scientists set out to discover what the thin lubricating layer was actually made of. The researchers scraped off and analyzed a bit of the lubricating layer from 7 metal hip implants removed from patients.

To their surprise, the scientists found that the layer was mostly made of graphitic carbon. They found very little protein at all.

With this knowledge, researchers can now aim to create safer, longer-lasting hip implants by encouraging carbon to stick to the metal joint.

“The results of such research could have important implications for several hundred thousand Americans who undergo hip replacement each year,” says Dr. Stephen I. Katz, director of NIH’s National Institute of Arthritis and Musculoskeletal and Skin Diseases.